1. Field
The present invention generally relates to an image forming apparatus, and more particularly to an image forming apparatus capable of efficiently controlling light radiation to read an image with a symmetrical light reflection system.
2. Discussion of the Background
A conventional background image forming apparatus such as a copying machine uses an image scanner to read an image of an original document. Such an image scanner generally use a light source having a length sufficient to cover a width of the original document to be read. The light source may be a fluorescent lamp such as, for example, a xenon arc lamp having a diameter of the order of 10 mm. In comparison with a halogen lamp, for example, the xenon arc lamp generally has a lower luminance but a wider light emitting area. Therefore, the xenon arc lamp emits a greater amount of light, resulting in a high light emission rate on an electrical power consumption.
The light emission amount is in proportion almost to an area having a fluorescent coating. Therefore, the light emission amount can be increased by increasing a diameter of the glass tube to enlarge the fluorescent coated area. This approach, however, results in upsizing of the image scanner.
FIG. 1 illustrates a major portion of an example image scanner 100 used in the conventional background image forming apparatus. FIG. 2 illustrates a structure of a xenon arc lamp used in the image scanner 100 of FIG. 1. As illustrated in FIG. 1, the image scanner 100 includes a xenon arc lamp 101, a reflector 102, a contact glass 103, and a mirror 104. The xenon arc lamp 101 includes a transparent glass tube 111 with a thickness of the order of from approximately 0.5 mm to approximately 1 mm. The transparent glass tube 111 includes an internal surface 112 covered with a fluorescent coating and an aperture 113 having an angle θ, and is filled with a xenon gas. The transparent glass tube 111 further includes a pair of electrodes 114 and 115 which are disposed at positions facing each other relative to a center of the transparent glass tube.
When an alternating voltage of a few hundred volts is applied to the pair of electrodes 114 and 115, an electric discharge is caused inside the glass tube. The transparent glass tube 111 generates a ultraviolet radiation when an electron running by the electric discharge collides with an atom of xenon inside the transparent glass tube 111. The ultraviolet rays then impinges on the fluorescent coating of the internal surface 112 and, at this moment, the fluorescent coating is energized to output a visible radiation which is discharged outside through the aperture 113. A part of the visible radiation goes through the aperture 113 to the reflector 102 and is reflected by the reflector 102 toward a point in an area a on the contact glass 103, as indicated by a line L1. Another part of the visible radiation goes through the aperture 113 directly to a point in the area a, as indicated by a line L2. Further another part of the visible radiation goes through the aperture 113 directly to a point in an area b on the contact glass 103. The light radiation to the area b is, however, undesirable.
The reflected light from the points in the area a is forwarded to the mirror 104 and is reflected by the mirror 104 toward other optical components (not shown), as indicated by a line L3. The light is finally directed to an imaging lens and an image pickup device such as a CCD (charge-coupled device) which reads the light as image information.
The xenon arc lamp, however, cannot generate a sufficient light amount in a case of a productivity and high-speed image forming apparatus such as a high-speed full-color scanner, for example, which needs a greater amount of light radiation to read images at a high speed. To increase a light radiation, it is needed to increase an area of the internal surface 112 covered with the fluorescent coating. This leads an increase of a diameter of the transparent glass tube 111 and also a size of the reflector 102, resulting in upsizing of the image scanner 100.